Ceramic matrix composites (CMC) are non-metallic materials that typically comprise ceramic fibers embedded in a composite matrix. CMCs are lightweight and exhibit markedly enhanced thermal strengths. These properties have made them attractive materials for component fabrication in many industries, such as the aerospace, automotive, and military equipment industries, where lightweight thermally resistant structures are desired. For example, CMCs continue to be explored for use in gas turbine engine applications to reduce the overall weight of the engine and improve engine efficiency and fuel savings. However, the strength and performance characteristics of CMCs may be dependent upon the integrity of the interfacial bond between the CMC component and any metallic component to which it is mated.
Due to the differing thermal characteristics of metallic and non-metallic materials such as CMCs, including for example the coefficient of thermal expansion (CTE), and the significant wear that can be caused by the relative motion of the two mating components, it is difficult to provide a robust method for attaching a metallic component to a non-metallic component. For example, in jet aircraft applications, for turbine blades, vanes etc., the metallic/non-metallic interface is not permanently affixed but rather experiences significant relative motion.
Thus the interfacial bond strength between the non-metallic component and the metallic component may be compromised upon exposure to high temperatures such as those experienced during some high-temperature engine operations, potentially leading to structural break-down of the component and possible in-service failure. To provide performance characteristics necessary for the safe use of CMCs in gas turbine engines and other applications, strategies are needed to improve the interfacial bond strength of the metallic and non-metallic components.
One possible solution is to provide a sacrificial layer between the metallic and non-metallic component that allows for some relative motion without damaging the two components. The challenge has been to provide a bonding layer of suitable material that is free floating but will remain between two components having different thermal properties. Transient liquid phase (TLP) and partial transient liquid phase (PTLP) bonding processes have been found to be useful alternatives to welding and brazing as ways to bond metals and non-metals such as CMCs.
The TLP bonding process generally involves placing one or more thin compliant interlayers of material between the materials to be bonded to form an assembly; heating the assembly to a first temperature to temporarily produce a “transient” liquid in the bonding region; and maintaining the assembly at a bonding temperature (which may be the same as the first temperature) until the liquid has isothermally solidified due to diffusion of the compliant material into the two components being joined. Holding the assembly at the bonding temperature creates a substantially homogeneous diffusion bond between the two materials. The resulting bond can be stronger than either of the two components alone. The interlayer(s) can be in many forms, including thin foil, powder, paste, vapor deposition, or electroplating. Pressure may be applied to the opposing materials, and various heat sources used, including radiation, conduction, and radio-frequency induction.
The main advantage of TLP bonding is that the resulting bond between the compliant material and the metallic component typically has a melting temperature above the temperature used for TLP bonding so that the formed bond may operate at temperatures well above the bonding temperature. This feature may be advantageous, for example, when joining temperature-sensitive metals whose microstructures could be damaged by too much thermal energy input. TLP bonding is often used in high-temperature applications where welding, brazing, and diffusion brazing cannot be used. The interlayer material may be any metallic material.
Partial transient liquid phase (PTLP) bonding is a variant of TLP typically used to join ceramics. In PTLP bonding, the interlayer may comprise thin layers of low-melting point metals or alloys on each side of a thicker refractory metallic layer. Among the advantages of PTLP bonding are the following: (1) The dual nature of the multi-layer interlayer combines some beneficial properties of brazing and diffusion bonding (2) Lower bonding temperatures can minimize thermally induced stresses.
PTLP bonding is often performed with elemental interlayers designed to eventually form a solid solution after isothermal solidification and subsequent homogenization steps. However, the resulting strength of the solid-solution bond may not be sufficient for certain applications, especially in the aerospace industry.
The present disclosure is directed to providing a means for attaching a metallic component to a non-metallic component such as a CMC using a compliant material having thermal properties intermediate those of the metallic component and the non-metallic component.